Network Working Group D. McGrew
Request for Comments: 4543 Cisco Systems, Inc.
Category: Standards Track J. Viega
McAfee, Inc.
May 2006
The Use of Galois Message Authentication Code (GMAC) inIPsec ESP and AH
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This memo describes the use of the Advanced Encryption Standard (AES)
Galois Message Authentication Code (GMAC) as a mechanism to provide
data origin authentication, but not confidentiality, within the IPsec
Encapsulating Security Payload (ESP) and Authentication Header (AH).
GMAC is based on the Galois/Counter Mode (GCM) of operation, and can
be efficiently implemented in hardware for speeds of 10 gigabits per
second and above, and is also well-suited to software
implementations.
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RFC 4543 GMAC in IPsec ESP and AH May 2006
Table of Contents
1. Introduction ....................................................21.1. Conventions Used in This Document ..........................32. AES-GMAC ........................................................33. The Use of AES-GMAC in ESP ......................................33.1. Initialization Vector ......................................43.2. Nonce Format ...............................................43.3. AAD Construction ...........................................53.4. Integrity Check Value (ICV) ................................63.5. Differences with AES-GCM-ESP ...............................63.6. Packet Expansion ...........................................74. The Use of AES-GMAC in AH .......................................75. IKE Conventions .................................................85.1. Phase 1 Identifier .........................................85.2. Phase 2 Identifier .........................................85.3. Key Length Attribute .......................................95.4. Keying Material and Salt Values ............................96. Test Vectors ....................................................97. Security Considerations ........................................108. Design Rationale ...............................................119. IANA Considerations ............................................1110. Acknowledgements ..............................................1111. References ....................................................1211.1. Normative References .....................................1211.2. Informative References ...................................121. Introduction
This document describes the use of AES-GMAC mode (AES-GMAC) as a
mechanism for data origin authentication in ESP [RFC4303] and AH
[RFC4302]. We refer to these methods as ENCR_NULL_AUTH_AES_GMAC and
AUTH_AES_GMAC, respectively. ENCR_NULL_AUTH_AES_GMAC is a companion
to the AES Galois/Counter Mode ESP [RFC4106], which provides
authentication as well as confidentiality. ENCR_NULL_AUTH_AES_GMAC
is intended for cases in which confidentiality is not desired. Like
GCM, GMAC is efficient and secure, and is amenable to high-speed
implementations in hardware. ENCR_NULL_AUTH_AES_GMAC and
AUTH_AES_GMAC are designed so that the incremental cost of
implementation, given an implementation is AES-GCM-ESP, is small.
This document does not cover implementation details of GCM or GMAC.
Those details can be found in [GCM], along with test vectors.
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RFC 4543 GMAC in IPsec ESP and AH May 20061.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. AES-GMAC
GMAC is a block cipher mode of operation providing data origin
authentication. It is defined in terms of the GCM authenticated
encryption operation as follows. The GCM authenticated encryption
operation has four inputs: a secret key, an initialization vector
(IV), a plaintext, and an input for additional authenticated data
(AAD). It has two outputs, a ciphertext whose length is identical to
the plaintext and an authentication tag. GMAC is the special case of
GCM in which the plaintext has a length of zero. The (zero-length)
ciphertext output is ignored, of course, so that the only output of
the function is the Authentication Tag. In the following, we
describe how the GMAC IV and AAD are formed from the ESP and AH
fields, and how the ESP and AH packets are formed from the
Authentication Tag.
Below we refer to the AES-GMAC IV input as a nonce, in order to
distinguish it from the IV fields in the packets. The same nonce and
key combination MUST NOT be used more than once, since reusing a
nonce/key combination destroys the security guarantees of AES-GMAC.
Because of this restriction, it can be difficult to use this mode
securely when using statically configured keys. For the sake of good
security, implementations MUST use an automated key management
system, such as the Internet Key Exchange (IKE) (either version two
[RFC4306] or version one [RFC2409]), to ensure that this requirement
is met.
3. The Use of AES-GMAC in ESP
The AES-GMAC algorithm for ESP is defined as an ESP "combined mode"
algorithm (see Section 3.2.3 of [RFC4303]), rather than an ESP
integrity algorithm. It is called ENCR_NULL_AUTH_AES_GMAC to
highlight the fact that it performs no encryption and provides no
confidentiality.
Rationale: ESP makes no provision for integrity transforms to
place an initialization vector within the Payload field; only
encryption transforms are expected to use IVs. Defining GMAC as
an encryption transform avoids this issue, and allows GMAC to
benefit from the same pipelining as does GCM.
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RFC 4543 GMAC in IPsec ESP and AH May 2006
Like all ESP combined modes, it is registered in IKEv2 as an
encryption transform, or "Type 1" transform. It MUST NOT be used in
conjunction with any other ESP encryption transform (within a
particular ESP encapsulation). If confidentiality is desired, then
GCM ESP [RFC4106] SHOULD be used instead.
3.1. Initialization Vector
With ENCR_NULL_AUTH_AES_GMAC, an explicit Initialization Vector (IV)
is included in the ESP Payload, at the outset of that field. The IV
MUST be eight octets long. For a given key, the IV MUST NOT repeat.
The most natural way to meet this requirement is to set the IV using
a counter, but implementations are free to set the IV field in any
way that guarantees uniqueness, such as a linear feedback shift
register (LFSR). Note that the sender can use any IV generation
method that meets the uniqueness requirement without coordinating
with the receiver.
3.2. Nonce Format
The nonce passed to the AES-GMAC authentication algorithm has the
following layout:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Salt |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initialization Vector |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Nonce Format
The components of the nonce are as follows:
Salt
The salt field is a four-octet value that is assigned at the
beginning of the security association, and then remains constant
for the life of the security association. The salt SHOULD be
unpredictable (i.e., chosen at random) before it is selected, but
need not be secret. We describe how to set the salt for a
Security Association established via the Internet Key Exchange in
Section 5.4.
Initialization Vector
The IV field is described in Section 3.1.
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RFC 4543 GMAC in IPsec ESP and AH May 2006
The use of 32-bit sequence numbers vs. 64-bit extended sequence
numbers is determined by the security association (SA) management
protocol that is used to create the SA. For IKEv2 [RFC4306] this is
negotiated via Transform Type 5, and the default for ESP is to use
64-bit extended sequence numbers in the absence of negotiation (e.g.,
see Section 2.2.1 of [RFC4303]).
3.4. Integrity Check Value (ICV)
The ICV consists solely of the AES-GMAC Authentication Tag. The
Authentication Tag MUST NOT be truncated, so the length of the ICV is
16 octets.
3.5. Differences with AES-GCM-ESP
In this section, we highlight the differences between this
specification and AES-GCM-ESP [RFC4106]. The essential difference is
that in this document, the AAD consists of the SPI, Sequence Number,
and ESP Payload, and the AES-GCM plaintext is zero-length, while in
AES-GCM-ESP, the AAD consists only of the SPI and Sequence Number,
and the AES-GCM plaintext consists of the ESP Payload. These
differences are illustrated in Figure 4. This figure shows the case
in which the Extended Sequence Number option is not used. When that
option is exercised, the Sequence Number field in the figure would be
replaced with the Extended Sequence Number.
Importantly, ENCR_NULL_AUTH_AES_GMAC is *not* equivalent to AES-GCM-
ESP with encryption "turned off". However, the ICV computations
performed in both cases are similar because of the structure of the
GHASH function [GCM].
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RFC 4543 GMAC in IPsec ESP and AH May 2006
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initialization Vector (IV) |
| (8 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Integrity Check Value (ICV) (16 octets) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: The AUTH_AES_GMAC Authentication Data Format
The IV is as described in Section 3.1. The Integrity Check Value
(ICV) is as described in Section 3.4.
The GMAC Nonce input is formed as described in Section 3.2. The GMAC
AAD input consists of the authenticated data as defined in Section3.1 of [RFC4302]. These values are provided as to that algorithm,
along with the secret key, and the resulting authentication tag given
as output is used to form the ICV.
5. IKE Conventions
This section describes the conventions used to generate keying
material and salt values for use with ENCR_NULL_AUTH_AES_GMAC and
AUTH_AES_GMAC using the Internet Key Exchange (IKE) versions one
[RFC2409] and two [RFC4306].
5.1. Phase 1 Identifier
This document does not specify the conventions for using AES-GMAC for
IKE Phase 1 negotiations. For AES-GMAC to be used in this manner, a
separate specification would be needed, and an Encryption Algorithm
Identifier would need to be assigned. Implementations SHOULD use an
IKE Phase 1 cipher that is at least as strong as AES-GMAC. The use
of AES-CBC [RFC3602] with the same AES key size as used by
ENCR_NULL_AUTH_AES_GMAC or AUTH_AES_GMAC is RECOMMENDED.
5.2. Phase 2 Identifier
For IKE Phase 2 negotiations, IANA has assigned identifiers as
described in Section 9.
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RFC 4543 GMAC in IPsec ESP and AH May 20065.3. Key Length Attribute
AES-GMAC can be used with any of the three AES key lengths. The way
that the key length is indicated is different for AH and ESP.
For AH, each key length has its own separate integrity transform
identifier and algorithm name (Section 9). The IKE Key Length
attribute MUST NOT be used with these identifiers. This transform
MUST NOT be used with ESP.
For ESP, there is a single encryption transform identifier (which
represents the combined transform) (Section 9). The IKE Key Length
attribute MUST be used with each use of this identifier to indicate
the key length. The Key Length attribute MUST have a value of 128,
192, or 256.
5.4. Keying Material and Salt Values
IKE makes use of a pseudo-random function (PRF) to derive keying
material. The PRF is used iteratively to derive keying material of
arbitrary size, called KEYMAT. Keying material is extracted from the
output string without regard to boundaries.
The size of the KEYMAT for the ENCR_NULL_AUTH_AES_GMAC and
AUTH_AES_GMAC MUST be four octets longer than is needed for the
associated AES key. The keying material is used as follows:
ENCR_NULL_AUTH_AES_GMAC with a 128-bit key and AUTH_AES_128_GMAC
The KEYMAT requested for each AES-GMAC key is 20 octets. The
first 16 octets are the 128-bit AES key, and the remaining four
octets are used as the salt value in the nonce.
ENCR_NULL_AUTH_AES_GMAC with a 192-bit key and AUTH_AES_192_GMAC
The KEYMAT requested for each AES-GMAC key is 28 octets. The
first 24 octets are the 192-bit AES key, and the remaining four
octets are used as the salt value in the nonce.
ENCR_NULL_AUTH_AES_GMAC with a 256-bit key and AUTH_AES_256_GMAC
The KEYMAT requested for each AES-GMAC key is 36 octets. The
first 32 octets are the 256-bit AES key, and the remaining four
octets are used as the salt value in the nonce.
6. Test VectorsAppendix B of [GCM] provides test vectors that will assist
implementers with AES-GMAC.
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RFC 4543 GMAC in IPsec ESP and AH May 20067. Security Considerations
Since the authentication coverage is different between AES-GCM-ESP
and this specification (see Figure 4), it is worth pointing out that
both specifications are secure. In ENCR_NULL_AUTH_AES_GMAC, the IV
is not included in either the plaintext or the additional
authenticated data. This does not adversely affect security, because
the IV field only provides an input to the GMAC IV, which is not
required to be authenticated (see [GCM]). In AUTH_AES_GMAC, the IV
is included in the additional authenticated data. This fact has no
adverse effect on security; it follows from the property that GMAC is
secure even against attacks in which the adversary can manipulate
both the IV and the message. Even an adversary with these powerful
capabilities cannot forge an authentication tag for any message
(other than one that was submitted to the chosen-message oracle).
Since such an adversary could easily choose messages that contain the
IVs with which they correspond, there are no security problems with
the inclusion of the IV in the AAD.
GMAC is provably secure against adversaries that can adaptively
choose plaintexts, ICVs and the AAD field, under standard
cryptographic assumptions (roughly, that the output of the underlying
cipher under a randomly chosen key is indistinguishable from a
randomly selected output). Essentially, this means that, if used
within its intended parameters, a break of GMAC implies a break of
the underlying block cipher. The proof of security is available in
[GCMP].
The most important security consideration is that the IV never
repeats for a given key. In part, this is handled by disallowing the
use of AES-GMAC when using statically configured keys, as discussed
in Section 2.
When IKE is used to establish fresh keys between two peer entities,
separate keys are established for the two traffic flows. If a
different mechanism is used to establish fresh keys, one that
establishes only a single key to protect packets, then there is a
high probability that the peers will select the same IV values for
some packets. Thus, to avoid counter block collisions, ESP or AH
implementations that permit use of the same key for protecting
packets with the same peer MUST ensure that the two peers assign
different salt values to the security association (SA).
The other consideration is that, as with any block cipher mode of
operation, the security of all data protected under a given security
association decreases slightly with each message.
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RFC 4543 GMAC in IPsec ESP and AH May 2006
To protect against this problem, implementations MUST generate a
fresh key before processing 2^64 blocks of data with a given key.
Note that it is impossible to reach this limit when using 32-bit
Sequence Numbers.
Note that, for each message, GMAC calls the block cipher only once.
8. Design Rationale
This specification was designed to be as similar to AES-GCM-ESP
[RFC4106] as possible. We re-use the design and implementation
experience from that specification. We include all three AES key
sizes since AES-GCM-ESP supports all of those sizes, and the larger
key sizes provide future users with more high-security options.
9. IANA Considerations
IANA has assigned the following IKEv2 parameters. For the use of AES
GMAC in AH, the following integrity (type 3) transform identifiers
have been assigned:
"9" for AUTH_AES_128_GMAC
"10" for AUTH_AES_192_GMAC
"11" for AUTH_AES_256_GMAC
For the use of AES-GMAC in ESP, the following encryption (type 1)
transform identifier has been assigned:
"21" for ENCR_NULL_AUTH_AES_GMAC
10. Acknowledgements
Our discussions with Fabio Maino and David Black significantly
improved this specification, and Tero Kivinen provided us with useful
comments. Steve Kent provided guidance on ESP interactions. This
work is closely modeled after AES-GCM, which itself is closely
modeled after Russ Housley's AES-CCM transform [RFC4309].
Additionally, the GCM mode of operation was originally conceived as
an improvement to the CWC mode [CWC] in which Doug Whiting and Yoshi
Kohno participated. We express our thanks to Fabio, David, Tero,
Steve, Russ, Doug, and Yoshi.
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RFC 4543 GMAC in IPsec ESP and AH May 2006
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